An LED drive circuit is known which drives LEDs to emit light by applying a full-wave rectified waveform obtained from an AC commercial power supply directly to an LED array. The LED array here is a series connection of a plurality of LED arrays and is constructed to be able to withstand a high voltage. Compared with other types of LED drive circuits that drive LEDs to emit light by generating a constant voltage from an AC commercial power supply, the above LED drive circuit has the advantage that the circuit configuration is simple and compact.
However, if a full-wave rectified waveform is simply applied to an LED array, a problem occurs in which the LEDs light only during the periods when the full-wave rectified waveform exceeds the threshold value of the LED array. For example, when the forward voltage Vf of each LED is 3 V, and the LED array is constructed by connecting 40 such LEDs in series, the threshold value of the LED array is 120 V. Suppose that the rms value of the AC commercial power supply is 100 V; then, with the above LED drive circuit, the LEDs light only during short periods when the full-wave rectified waveform exceeds 120 V. As a result, with the above LED drive circuit, not only do the LEDs become dim or produce perceivable flicker, but the power factor and distortion factor also decrease.
To address this, it is known to provide methods for extending the LED ON period; in one known method, the LED array is divided into a number of LED sub-arrays, and provisions are made to turn on only some of the LED sub-arrays during the low voltage period of the full-wave rectified waveform and to increase the number of LED sub-arrays to be turned on as the voltage increases (refer to patent document 1).
FIG. 4 is a circuit diagram showing one example of an LED drive circuit; the diagram is redrawn from FIG. 26 given in patent document 1 without departing from the purpose thereof. In the diagram, the current-limiting resistor is replaced by a current-limiting circuit. Therefore, it should be noted that the LED drive circuit shown in FIG. 4 is given only for illustrative purposes and does not directly represent the LED drive circuit known in the art.
In the example of FIG. 4, a bypass circuit which turns on (conducts) during the low voltage period of the full-wave rectified waveform and turns off (does not conduct) during the high voltage period is connected to a connection node between each LED sub-array. The bypass circuit is controlled on and off according to the voltage value of the full-wave rectified waveform or the value of the current that flows through the LED array.
The LED drive circuit 400 shown in FIG. 4 comprises a diode bridge circuit 405, LED sub-arrays 410 and 430, a bypass circuit 420, and a current-limiting resistor 440. A commercial power supply 406 is connected to input terminals of the diode bridge circuit 405.
The bridge rectifier circuit 405 is constructed from four diodes 401, 402, 403, and 404, and has a terminal A as an output terminal for outputting a full-wave rectified waveform and a terminal B as a terminal for providing a reference voltage. The LED sub-array 410 is constructed from a series connection of a large number of LEDs including LEDs 411 and 412. Similarly, the LED sub-array 430 is constructed from a series connection of a large number of LEDs including LEDs 431 and 432. The bypass circuit 420 includes a pull-up resistor 421, a current-sensing resistor 424, a field-effect transistor (FET) 422, a bipolar transistor (hereinafter simply “transistor”) 423, a first current input terminal 427, a second current input terminal 428, and a current output terminal 429. Similarly to the bypass circuit 420, the current-limiting circuit 440 includes a pull-up resistor 441, a current-sensing resistor 444, an FET 442, a transistor 443, a current input terminal 447, and a current output terminal 449. The FETs 422 and 442 are enhancement-mode n-type MOS-FETs.
FIG. 5(a) is a waveform diagram depicting a full-wave rectified waveform, and FIG. 5(b) is a waveform diagram depicting a circuit current I in the LED drive circuit 400. The same time axis is used for both FIGS. 5 (a) and 5(b).
In FIG. 5, no circuit current I flows during the period t1 because the voltage value of the full-wave rectified waveform is smaller than the threshold value of the LED sub-array 410.
In the period t2, the voltage value of the full-wave rectified waveform exceeds the threshold value of the LED sub-array 410 but is smaller than the sum of the threshold value of the LED sub-array 410 and the threshold value of the LED sub-array 430. In this case, the circuit current I passes through the bypass circuit 420 and returns to the bridge rectifier circuit 405. During the period t2, feedback is applied so as to maintain the base-emitter voltage of the transistor 423 at 0.6 V, and the bypass circuit 420 thus operates in a constant current mode. Actually, in the last short portion of the period t2, the voltage value of the full-wave rectified waveform becomes slightly larger than the sum of the threshold value of the LED sub-array 410 and the threshold value of the LED sub-array 430, and a current flows in from the LED sub-array 430 via the current input terminal 428.
In the period t3, the voltage value of the full-wave rectified waveform exceeds the sum of the threshold value of the LED sub-array 410 and the threshold value of the LED sub-array 430, and the current flows to the current input terminal 428 by passing through the LED sub-array 430. At this time, the transistor 423 saturates, and the gate voltage of the FET 422 becomes equal to the reference voltage (the voltage at the terminal B), so that the FET 422 is cut off. As a result, the current flowing into the bypass circuit 420 via the current input terminal 427 rapidly drops. On the other hand, in the current-limiting circuit 440, feedback is applied so as to maintain the base-emitter voltage of the transistor 443 at 0.6 V, and the current-limiting circuit 440 thus operates in a constant current mode. The process that takes place during the period that the voltage of the full-wave rectified waveform falls is the reverse of the process that takes place during the period that the voltage of the full-wave rectified waveform rises.